A problem frequently encountered by users of a computer system that utilizes a direct access storage device (DASD) occurs when space constraints prevent a data set from being initially allocated or extended on the storage device. This problem occurs because the user must estimate the maximum size of the data set to be stored on the direct access storage device system. If the system cannot allocate as much space as the user has requested, a space allocation error is returned to the user.
In many types of data set organizations, when a space allocation request is made, it must be allocated within a preset number of extents or sections. For example, a five extent limit means that a space request (e.g. one hundred cylinders) cannot span more than five extents or contiguous sections. This type of organizational constraint is a frequent cause of space allocation errors.